The invention relates to a signal acquisition and distance variation measurement system for laser ranging interferometers, in particular for use in a space application, having a signal acquisition unit.
In the past, spaceborne laser terminals have been employed only for communication purposes. However, investigations have been made to use laser instruments as radio instruments due to their reliability. As a result, future planned missions such as eLISA, NGO and, next generation geodesy missions are going to implement laser technology in order to measure distance variations between satellites of a satellite constellation as the interference of a reference beam with a measurement beam allows performing range measurements with a precision equal to a fraction of the laser wavelength. The range measurement relies on a tacking phasemeter and on the accuracy with which it is able to evaluate the phase of the interference signal.
By means of a satellite constellation, gravitational waves and planet gravimetry can be studied, as their orbital trajectories are influenced by these physical phenomena. A technique used to evaluate trajectory anomalies introduced by gravimetry or by gravitational waves is measuring the phase difference of an electromagnetic signal between a transmitting-receiving-satellite. This technique has been employed by the Gravity Recovery And Climate Experiment (GRACE) which addresses its measurements through a microwave link between a trailing and a following satellite. It is known that measurement accuracy can be highly improved using a laser link rather than a microwave link.
The accuracy of the laser range measurement relies on a phasemeter, mainly on a PLL (Phase-Locked-Loop), which measures the phase difference between a measurement beam and a reference beam. The main architecture of phasemeters is based on so-called mixing phase detector PLLs as they exploit the most accurate phase measurement when compared to other PLL concepts. The drawback of these PLLs is the small pull-in-ranges which constrains the maximum frequency offset allowable between the input signal and the VCO (Voltage Controlled Oscillator).
Before establishing a measurement link, the satellite constellation has to undergo an initial signal acquisition phase. Independently from the complexity of this mandatory process, which can vary according to the mission and/or instrument layout, the initial frequency offset between the input signal and the VCO can reach up to 200 MHz or more. The phasemeter requires a build-in frequency estimation stage algorithm, respectively, in order to properly steer the VCO frequency of the main PLL towards the incoming beatnote.
From prior art, the importance of the main PLL in addressing the range measurement accuracy is known. In this sense, efforts have been made in developing phasemeter prototypes and signal processing algorithms in order to fulfill a specific mission requirement.
A more broadband view on signal processing for the initial signal acquisition may be made by using FFT (Fast Fourier Transformation) or DFFT (Digital Fast Fourier Transformation) based algorithms. These algorithms, working in a frequency domain, require, in addition to the main PLL board, processing units where FFT algorithms are implemented. These are used to initialize the main PLL and lock the incoming measurement signal (Laser Light Signal). FFT based algorithms can also be directly implemented in FPGAs (Field Programmable Gate Array). Nevertheless, the amount of FPGA logic elements and memory required by these algorithms in order to detect with sufficient accuracy the correct frequency of the received signal in a reasonable amount of time makes it almost impossible to implement a full phasemeter in just a single FPGA.
The inference of the phasemeter detection band is implicitly designated via the mandatory signal acquisition phase. Until now, no dedicated autonomous process for the inference of the phasemeter detection band is known.